This invention relates to a method and an apparatus for measuring load characteristics of a microwave transistor, particularly, a microwave power transistor.
It is essential to know the large-signal characteristics of the transistors to estimate microwave transistor elements and design circuits for microwave transistors. Being dependent on the input signal level and load, large-signal characterization of large-signal power transistors is difficult compared with characterization of linear, small-signal transistors. S-parameter characterization, adequate for a small-signal element, is not always effective for a large-signal element. Consequently, characteristics of a large-signal element are generally measured by the use of a method known as load-pull characterization wherein the relation between load impedance and output power is measured with the load varied by the use of an output impedance tuner. With this method, it is indispensable to measure the load impedance at every point of measurement by disconnecting the tuner. Reconnection of the tuner is not sufficiently reproducible and unavoidably offers a problem of precision. When the tuner is calibrated beforehand, another problem arises in that the mechanism of the tuner is inadequate to reliably reproducing the impedance. For higher-frequency power transistors, which inevitably have low input and output impedances, it becomes more difficult to manufacture a low-loss output impedance tuner with excellent impedance reproducibility. Measurement of the load characteristic thus poses various problems, particularly, for low-impedance microwave power transistors.